Method of electrically heating and welding glass elements



United States Patent METHOD OF- ELECTRICALLY HEATING AND WELDIYG GLASSELEMENTS poration of Pennsylvania N0 Drawing. Application April 30,1952, Serial No. 285,333

14 Claims. (Cl. 4982) The present invention relates to an electricallyconductive solution and it has particular relation to the provision of.an electrically conductive solution of colloidal graphite suitable foruse in the production of all glass welded double glazed units.

It is known that welded double glazed units may be prepared by applyinga stripe of an electrically conductive materialsuch as aqueous colloidalgraphite along the marginal edges of at least one of two sheets ofglass, preheating the two sheets of glass, supporting the striped sheetin horizontal position above the other sheet, electrically heating themarginal edges of the glass sheets by passing an electric currentthrough the stripes to generate heat sufficient to soften the contiguousglass and cause the edge portions of the upper sheet to drop down andweld to the marginal edges of the lower sheet. Suitable commerciallyavailable colloidal graphite solutions usually contain a small amount ofsodium silicate.

One such method of forming welded glass double glazed units is describedin U. S. Patents Nos. 2,389,360 and 2,394,051. A later and improvedtechnique is disclosed in an application of William R. Clever and Harry0. Phalin filed March 14, 1950, Serial No. 149,562, now Patent No.2,624,979 dated January 13, 1953, and entitled Welded Double GlazingUnit, now U. S. Patent No. 2,624,979.

In, the commercial production of welded double glazed units according tosuch methods, the stripe of colloidal graphite is applied to the glasssheet by means of a wheel and cup striping unit which comprises astriping wheel which rotates in a reservoir containing the colloidalgraphite solution. The coated wheel deposits a stripe of the colloidalgraphite solution on the surface of the glass sheet when the glass sheetand coated wheel are brought into contact with each other. Such astriping mechanism is described in a copending application of JohnKallenborn, filed January 10, 1947, Serial No. 721,285, now Patent No.2,597,106, dated May 20, 1952, entitled Aquadag Stripe Applicator, nowU. S. Patent No. 2,597,106.

In using such a mechanism to apply a stripe of electrically conductivematerial to a surface of a sheet of glass it can be seen that theviscosity of the solution is an important factor. One method which hasbeen employed to control the viscosity of the striping solution is toadd and aqueous solution of ammonium alignate to the aqueous solution ofcolloidal graphite. However, the use of ammonium alginate in combinationwith colloidal graphite has been unsatisfactory, both with respect tothe physical uniformity of the stripe which is applied and with respectto the electrical resistance of the stripe, and frequent shut downs anddelays in production have occurred. Other compounds used in combinationwith colloidal graphite which have not been satisfactory include ethylcellulose, methyl cellulose, and polyvinyl butyral dissolved indenatured ethyl alcohol.

In accordance with the present invention an, electrically conductivesolution has been provided which is suitable for use in the productionof all glass welded double glazed units. This electrically conductivesolution comprises an aqueous solution of colloidal graphite and analkali metal carboxyalkyl cellulose such as lithium, potassium or sodiumcarboxymethyl cellulose. The alkyl substituent may be methyl, ethylpropyl, isopropyl, butyl and higher alkyl radicals. The use of such anelectrically conductive solution has resulted in production of moreuniform stripes whose preheating, electrical resistance and weldingqualities are excellent and easily reproducible. It has also been foundthat the storage in sealed containers of such striping solutions attemperatures just above their freezing temperature up to 60 F. withoccasional agitation is beneficial in providing for the production ofelectrically conducting stripes of uniform thickness and electricalresistance.

In accordance with a further embodiment of this invention, it has beenfound that it is advantageous to use different formulations of theaqueous solution of colloidal graphite and sodium carboxymethylcellulose when a double glazed unit of differing edge dimensions isbeing produced. For example, in the production of a rectangular doubleglazed unit, the ratio of colloidal graphite to sodium carboxymethylcellulose in the solution should be greater for the stripes along thelonger sides of the glass sheet than for the stripes along the shortersides of the glass.

In the practice of the invention, an electrically conductive solution ofcolloidal graphite and sodium carboxymethyl cellulose is prepared in thefollowing manner. Ten grams of sodium carboxymethyl cellulose isthoroughly mixed with 3000 cubic centimeters of distilled water. Sodiumcarboxymethyl cellulose is prepared by treating alkali cellulose withsodium monochloroacetate and is commercially available in variousviscosities. The above formulation is prepared with sodium carboxymethylcellulose of high viscosity grade.

The viscosity of the mixture of water and sodium carboxymethyl celluloseis measured at 25 C. and adjusted so as to be within desired viscositylimits. A viscosity range of 70 to centipoises is preferred, but theappropriate viscosity is dependent upon many factors, such as therelative amount of striping solution which is added, the diameter of thestriping wheel, conformation of the rim of the striping wheel, rate ofrotation of the striping wheel, relative linear rates of travel of theglass sheets and striping wheels and many other factors. If theviscosity is too high, distilled water may be added to reduce theviscosity of the mixture. The addition of about 300 cubic centimeters ofdistilled water to 3000 cubic centimeters of solution as prepared abovehas been found to lower the viscosity of the solution about 10centipoises.

As stated above, it has been discovered that optimum welding is achievedwhen the electrically conductive solution of colloidal graphite andsodium carboxymethyl cellulose is varied as to the amounts of eachconstituent as the length of the stripe to be applied varies.Accordingly, the following formulations have been found to produceoptimum results when employed in the production of all glass weldeddouble glazed units as hereinafter more fully described. The table belowsets forth the amount of an aqueous solution of colloidal graphitecontaining 2 to 3 per cent by weight of graphite which is added to cubiccentimeters of a solution of sodium carboxymethyl cellulose prepared asdescribed above to form electrically conductive stripes of varyinglengths.

It is important that the colloidal graphite solution, the sodiumcarboxymethyl cellulose and the electrically conductive solution madetherefrom be kept at certain temperatures in order to provide anelectrically conductive material which will act with maximum efficiencyduring the welding operation. It has been found that optimum results areobtained when the electrically conductive solution and ingredientstherefore are stored at a temperature just above their freezingtemperature up to 60 F. The viscosity of the electrically conductivesolution when prepared and kept in this manner is very stable. Such aproperty is important when it is desired to formulate an electricallyconducting solution which can be used in production extending over aperiod of time.

Various techniques may be employed to apply the electrically conductingstripes to the glass. For example, the stripes may be applied bypainting, brushing, spraying screening or by other methods. One methodwhich has been found to be superior to other known methods is to applythe stripes by means of a wheel which is supported in a reservoircontaining the electrically conductive solution. This wheel may beindependently driven, or may be caused to rotate by contact with theglass to provide a continuous electrically conductive stripe on thesurface of the glass sheet. Said contact rotation of the wheel may becaused either by its movement across the surface of the glass or bymovement of the glass sheet past the wheel or a combination of suchmovements. A suitable apparatus for applying the stripes of electricallyconductive material to the glass sheet is described in the patentgranted to John Kallenborn mentioned above.

It is extremely important that the glass to which the electricallyconductive stripe is to be applied is as clean as possible. Any foreignsubstances on the surface of the glass in the area of the stripe are aptto cause irregular a when the temperature of the glass during thestriping operation is within a range of 200 to 350 F.

After the margins of at least one of the glass sheets have been stripedas desired with the electricity conducting solution of colloidalgraphite and sodium carboxymethyl cellulose, the two sheets of glass tobe welded are placed in a preheating furnace which brings the glass upto a temperature of approximately 1000 to 1100" F. preparatory towelding. This preheating constitutes a severe, but fundamental test ofthe suitability of any electrically conductive stripe. During thispreheating, virtually all of the sodium carboxymethyl cellulose and anyother organic constituent burn out leaving a graphite strip whoseelectrical characteristics approach those of a metallic ribbon ofgreatly reduced resistance. A uniform, complete removal of the sodiumcarboxymethyl cellulose is highly desirable. The air dried resistance ofa colloidal graphite stripe should be such that after preheating, theresistance lies within the approximate range of 0.001 to 0.003 megohmper inch.

When the sheets of glass have been preheated at ap- Cir proximately 1100F. for about 1 minute. they are then arranged in superposed relation andwelded as described in U. S. Patents Nos. 2,389,360 and 2,394,051 andthe patent granted to Clever et a1. disclosed above. The preferredwelding process requires nine heating cycles to complete the weld. Oneheating cycle comprises applying current to one pair of opposingparallel stripes and then to the other. The current is applied to thestripes by means of suitable electrodes positioned at each corner of theglass sheet as shown in the patents and pending application. Startingwith an initial temperature of 1100 F., the temperature of the glass isgradually and substantially uniformly raised to about 1900 to 2000 F.This increase in temperature reaches a maximum in the area of thestripes of colloidal graphite, and is progressively lower towards thecenter of the sheets of glass. This maximum temperature is reached onlyafter the stripes of graphite have been burned off and portions of themarginal edges of the upper sheet have softened and sagged down toengage the marginal edges of the lower sheet of glass.

After the glass sheets have been welded together to form the doubleglazed unit, the unit is annealed and cooled to room temperature. Thenthe moist air within the unit is purged by means of a dry humidified gasand the unit is sealed. Usually the units are prestressed prior to thesealing operation.

The use of an electrically conducting solution of colloidal graphite andsodium carboxymethyl cellulose has made possible the production ofdouble glazed units on a commercial scale without the numerousproduction breakdowns caused by failure of the electrically conductingstripe during the initial stages of the welding operation. A greaterdegree of heating as well as a more uniform degree of heating of themarginal edges of the glas sheet containing the electrically conductingstripe during the early stages of the welding operation has been madepossible by the use of such solution.

The graphite stripe as applied to the glass in combination with sodiumcarboxymethyl cellulose seems by visual observation to carry the heatinto the glass better than when applied alone as an aqueous solution ofcolloidal graphite or in combination with any other compound which hasbeen tested. On explanation of this might be that the particles ofgraphite are so aligned as to conduct current and pass heat more readilyinto the glass directly beneath and surrounding the stripe. The patternof consistent decomposition of the sodium carboxymethyl cellulose tovolatiles may contribute to the achievement of the proper alignment ofthe graphite particles.

Although the present invention has been described with reference tospecific details of certain embodiments thereof, it is not intended thatsuch details shall be regarded as limitations upon the scope of theinvention except insofar as included in the accompanying claims.

We claim:

1. A method of producing an electrically welded double glazed unit whichcomprises providing two fiat glass sheets, applying stripes of anelectrically conducting solution comprising an aqueous solution ofcolloidal graphite and an alkali metal carboxyalkyl cellulose to themarginal edges of one of said glass sheets, the ratio of colloidalgraphite to an alkali metal carboxyalkyl cellulose in the solution beinggreater as the length of stripe to be applied to the surface of theglass sheet is greater, preheating the glass sheets, superposing theglass sheets in spaced relation with the glass sheet containing theelectrically conductive stripes being above the other sheet of glass,electrically heating the margins of the glass sheets to cause them tosoften and weld together to form a double glazed unit and annealing theunit.

2. A method of producing an electrically welded double glazed unit whichcomprises providing two flat glass sheets, applying stripes of anelectrically conducting solution comprising an aqueous solution ofcolloidal graphite and sodium carboxymethyl cellulose to the marginaledges of one of said glass sheets, the ratio of colloidal graphite tosodium carboxymethyl cefiulose in the solution being greater as thelength of stripe to be applied to the surface of the glass sheet isgreater, preheating the glass sheets, superposing the glass sheets inspaced relation with the glass sheet containing the electricallyconductive stripes being above the other sheet of glass, electricallyheating the margins of the glass sheets to cause them to soften and weldtogether to form a double glazed unit and annealing the unit.

3. A method of producing an electrically welded rectangular doubleglazed unit which comprises providing two recta'ngularly shaped fiatglass sheets, applying stripes of an electrically conducting solutioncomprising an aqueous solution of colloidal graphite and sodiumcarboxymethyl cellulose to the marginal edges of one of said glasssheets, the ratio of colloidal graphite to sodium carboxymethylcellulose being greater for the two longer opposing parallel edgestripes than for the shorter opposing parallel edge stripes, preheatingthe glass, superposing the glass sheets in spaced relation with theglass sheet containing the electrically conductive stripes being abovethe other sheet of glass, electrically heating the margins of the upperglass sheet to cause them to soften and weld to the marginal edges ofthe lower glass sheet to form a double glazed unit and annealing theunit.

4. In the method of producing an electrically welded double glazed unitby applying stripes of an electrically conducting material to themarginal edges of a glass sheet, superposing the striped glass sheet inspaced relation to another glass sheet and electrically heating themarginal edges of the glass sheets to cause them to weld together, thestep which comprises applying stripes of an electrically conductingsolution comprising an aqueous solution of colloidal graphite and analkali metal carboxyalkyl cellulose to the marginal deges of the glasssheet.

5. In the method of producing an electrically welded double glazed unitby applying stripes of an electrically conducting material to themarginal edges of a glass sheet, superposing the striped glass sheet inspaced relation to another glass sheet and electrically heating themarginal edges of the glass sheets to cause them to weld together, thestep which comprises applying stripes of an electrically conductingsolution comprising an aqueous solution of colloidal graphite and sodiumcarboxymethyl cellulose to the marginal edges of the glass sheet.

6. In the method of producing an electrically welded double glazed unitby applying stripes of an electrically conducting material to themarginal edges of a glass sheet, superposing the striped glass sheet inspaced relation to another glass sheet and electrically heating themarginal edges of the glass sheets to cause them to weld together, thestep which comprises applying stripes of an electrically conductingsolution comprising an aqueous solution of colloidal graphite and sodiumcarboxymethyl cellulose to the marginal edges of the glass sheet, theratio of colloidal graphite to sodium carboxymethyl cellulose in thesolution being greater as the length of stripe to be applied to thesurface of the glass sheet is greater.

7. In the method of producing an electrically welded double glazed unitcomprising applying stripes of an electrically conducting material tothe marginal edges of a glass sheet, positioning the striped glass sheetin spaced relation to another glass sheet and electrically heating themarginal edges of the glass sheets to cause them to weld together, thestep which comprises applying stripes of a solution comprising colloidalgraphite and an alkali metal carboxyalkyl cellulose to the marginaledges of the glass sheet.

8. A method of producing an electrically welded multiple glazed unitwhich comprises applying to the marginal edges of at least one of theglass members forming the unit stripes of a solution comprisingcolloidal graphite and an alkali metal carboxyalkyl cellulose,positioning the striped member in spaced relation to another member andelectrically heating the marginal edges of the members to cause them toweld together and form a multiple glazed unit.

9. The method described in claim 8 wherein the alkali metal carboxyalkylcellulose is sodium carboxymethyl cellulose.

10. The method as described in claim 8 wherein the glass member is at atemperature between 200 to 350 F. at the time of application of thestripe of alkali metal carboxyalkyl cellulose.

11. A method of heating glass in a restricted area which comprisesapplying a solution comprising colloidal graphite and an alkali metalcarboxyalkyl cellulose to the restricted area of the glass and passingelectric current through the graphite to heat it and the glass adjacentthereto.

12. The method of claim 11 wherein the alkali metal carboxyalkylcellulose is sodium carboxymethyl cellulose.

13. A method of producing an electrically welded glass structure whichcomprises applying a solution comprising an alkali metal carboxyalkylcellulose and colloidal graphite to a restricted area on at least one ofthe glass members to form the structure, passing an electric currentthrough the graphite to heat it and the glass adjacent thereto andcontacting the heated glass with another member of the structure to weldthem together.

14. The method described in claim 13 wherein the alkali metalcarboxyalkyl cellulose is sodium carboxymethyl cellulose.

References Cited in the file of this patent UNITED STATES PATENTS2,176,879 Bartell Oct. 24, 1939 2,389,360 Guyer et al Nov. 20, 19452,394,051 Guyer et al Feb. 5, 1946 2,445,374 Van Wyck July 20, 19482,570,827 Madison et al Oct. 9, 1951 FOREIGN PATENTS 665,474 GermanySept. 26, 1938 OTHER REFERENCES Hercules, Sodium CarboxymethylCellulose, 1944, page 4.

